The basic function of a locomotion simulation device is to apply forces to an operator's body without having the body move. This allows the system to present the illusion of motion activities while maintaining a net body acceleration of zero. Another function is to take energy out of the operator This should be done in a manner that extracts a realistic amount of energy (as compared to performing the task in a real, as opposed to a virtual, environment). For training applications, there is an additional requirement that the activity is performed so as to realize positive training transfer. A preferred implementation might also include the following activities: moving naturally, moving across large areas, simulating ground features, simulating terrain features, climbing stairs, moving with assured safety, assuming alternative postures, climbing over/through obstacles, manipulating gear and interacting with vertical features.
Given these sets of requirements, there exists three categories of devices that can be employed for locomotion simulation: treadmills, whole-body exoskeletons and foot-haptic devices. The approach described herein is the foot-haptic approach, since the other approaches have significant shortcomings which limit their viability.
The selection analysis presented can be succinctly summarized, as shown in Table 1, below:
TABLE 1 ______________________________________ Ability to Satisfy System Ease of Type of Device Requirement Implementation ______________________________________ Treadmill Low High Foot Haptic Medium Medium Exoskeleton High Low ______________________________________
Treadmill concepts, for instance, lack the ability to simulate terrain and ground features. Since it is difficult to modulate the surface of the treadmill, they cannot exert a downward force on the soldiers feet, and the presentation of terrain features, such as stairs, is problematical. Treadmills are also not as adept at presenting vertical features to the operator, as compared to the exoskeleton and foot-haptic approaches. The primary reasons for this is that the exact location of the operator is not known precisely, thus increasing the risk of accidentally impacting the operator. Also, the distance the vertical features need to be moved in order to be brought into position is typically large, thus higher speeds would be required to move them into place in a given period of time.
The ability to support on-ground activities, such as crawling and rolling is limited by two factors: the size of the available surface, and safety considerations. The foot-haptic display does not support rolling as well as treadmills because the available surface is somewhat limited. However, since rolling is typically not carried out for an extended period under most conceivable scenarios, this is not a significant limitation. Although treadmills would seem to excel at providing this capability, certain designs are inadequate due to high force and mechanical closing gaps that pose a considerable risk of injury to the operator during on-ground activities.
Certain treadmill concepts are also excessively expensive, while others do not satisfy certain safety criteria, such as minimizing kinetic energies and closing gaps. Other concepts impart unwanted high-speed motion to the operator, possibly causing motion sickness and certainly not modeling reality. Certain treadmill designs require a very large area, on the order of 30 meters in diameter. A number of concepts are also technically unfeasible, due to material limitations, high power requirements, and other factors.
One device that comes close to satisfying the requirements for a whole-body kinesthetic display is a gimbal-mounted exoskeleton. However, such a device cannot be currently manufactured due to certain fundamental limitations (such as motor power to weight ratios) and certain programmatic considerations. For these reasons, no device of this complexity has ever been built and/or controlled). While it may be possible build and control such devices in the future, they would certainly be very costly, due to the high number of actuated degrees of freedom required. There are also safety concerns, since many of the actuators would need to be capable of suspending the mass of the body within the gimbal, which implies that they would be equally capable of exerting sufficient force to injure the operator.